1. Field of the Invention
The present invention relates to an insulation gate type semiconductor device such as a thin film transistor (TFT) having a thin film active layer (i.e., an activated region or a channel region) formed on an insulation substrate. A field to which the invention pertains is a semiconductor integrated circuit, a liquid crystal display device, an optical reading device or the like.
2. Description of the Prior Art
Recently, researches and developments have been made as to insulation gate type semiconductor devices having thin film active layers on insulation substrates. In particular, continuous efforts have been made on so-called thin film transistors (TFTs). These TFTs are intended to be used for controlling respective image elements of matrix structure in a display device such as an LCD. Depending upon a material to be used and a crystalline condition of the semiconductors, TFTs are classified into amorphous silicon TFTs and polycrystal silicon TFTs. However, recently, a material having an intermediate condition between the polycrystalline condition and amorphous condition has been studied. This is called a semi-amorphous condition and is considered as a condition where small crystals are floated on an amorphous formation.
Also, in a single crystal silicon IC, a polycrystal silicon TFT is used as a so-called SOI technique. For instance, this is used as a load transistor in a highly integrated SRAM. However, in this case, an amorphous silicon TFT is hardly used.
In general, an electric field mobility of a semiconductor under the amorphous condition is small, and it is therefore impossible to use the semiconductor as TFTs which need high speed operation. Also, in the amorphous silicon, the electric field mobility of P-type is small, and it is impossible to produce a P-channel type TFT (TFT of PMOS). Accordingly, it is impossible to form a complementary MOS circuit (CMOS) in combination with N-channel type TFT (TFT of NMOS).
However, TFTs formed of amorphous semiconductors have a feature that their OFF current is small. Therefore, such TFTs have been used where an extremely high speed operation is not needed like a liquid crystal active matrix transistor, one-way conductive type TFTs may be satisfactorily used and TFTs having a high charge holding capacity are needed.
On the other hand, a polycrystal semiconductor has a larger electric field mobility than that of an amorphous semiconductor. Therefore, in this case, it is possible to effect high speed operation. For example, with TFTs using a silicon film recrystallized through a laser anneal technique, it is possible to obtain a large electric field mobility of 300 cm2/Vs. This value is considered very high in view of the fact that the electric field mobility of a regular MOS transistor formed on a single crystal silicon substrate is approximately 500 cm2/Vs. In addition, the operation speed of the MOS circuit on the single crystal silicon substrate is considerably limited by an inherent capacitance between the substrate and wirings. In contrast, since the TFT is located on the insulation substrate, such a limitation is no longer needed and a considerably high speed operation is expected.
Also, it is possible to obtain PTFTs as well as NTFTs from polycrystal silicon, and hence it is possible to form a CMOS circuit thereby. For example, in a liquid crystal display device, a so-called monolithic structure is known in which not only active matrix portions but also peripheral circuits (such as drivers or the like) are formed by polycrystal CMOS TFTs. This point is noticed also in the TFTs used in the aforesaid SRAMs. In this case, PMOSs are formed by TFTs and are used as a load transistor.
However, in general, the polycrystal TFTs have an increased leak current and a poor performance of holding the electric charge of image elements of the active matrix since the electric field mobility of the polycrystal TFTs is larger than that of amorphous TFTs. For example, in the case where the polycrystal TFTs are used as the liquid crystal display elements, since conventionally, the size of the image elements is several hundreds of micrometers square and the image element capacities are large, there have been no serious problems. However, recently, the fine image elements have been used in accordance with a high resolution, and the image element capacities become small. The conventional image elements would be insufficient for stable static display.
There have been several solutions for the current leakage problems inherent in such polycrystal TFTs. One of the methods is to thin an active layer. It is reported that the OFF current would be small by the method. For instance, it is known that a thickness of the active layer is 25 nm whereby the OFF current might be less than 10xe2x88x9213A. It would be however very difficult to crystallize a thin semiconductor film and it is actually known that the thin semiconductor film could not easily be crystallized.
The method in which the active layer is thinned leads to the phenomenon in which a source/drain region is thinned. This is because the semiconductor film is formed so that the source/drain region is produced simultaneously with the formation of the active layer in accordance with a conventional production method and the source/drain region and the active layer have the same thickness. This would also lead to the increased resistance of the source/drain region.
For this reason, a method is used in which a thickness of almost all the source/drain region is increased. This means that a mask process is additionally used. This is undesired from the view point of productive yield.
Also, according to the present inventors"" knowledge, in the TFTs where a thickness of the active layer is 50 nm or less, a MOS threshold voltage is largely shifted, and this phenomenon is remarkable in case of NMOS""s. The threshold voltage would be zero or negative values. If, thus, the CMOS is formed by the TFTs, the operation would be unstable.
On the other hand, if the thickness of the active layer would be increased, the leakage current would be increased. The magnitude thereof is not in proportion to the thickness of the active layer. It is therefore reasonable that the leakage current would be increased in a non-linear manner due to some causes. The present inventors have studies and found that almost all the leakage current of the TFTs where the active layer is thick may flow through a part of the active layer on the substrate side in a bypass fashion. Two causes thereof might be found out. One cause is that there is a charge fixed to an interface energetic position between the substrate and the active layer. The other cause is that movable ions such as sodium or the like enter from the substrate into the active layers to thereby make conductive the part of the active layer on the substrate side. The later cause may be overcome by increasing a performance of the cleaning process.
However, whatever the interface between the substrate and the active layer was made clean, it was impossible to overcome the problem of the former cause. For example, the direct formation of the active layer on the substrate would lead to raising the interface energetic position. Accordingly, it was impossible to obviate the problem of the leakage current even if an oxide layer (such as heat oxide film of silicon) having a high quality to the same extent as that of the gate oxide film was used as an underlayer and the active layer was formed thereon. Namely, it has been found that it is difficult to remove the fixed charge.
In order to solve the above-noted defects or difficulties, according to the present invention, an additional gate electrode (hereinafter referred to as a bottom side gate electrode) is formed between a substrate and an active layer, and this gate electrode is kept at a suitable potential whereby the stationary charge described above may be cancelled.
According to the present invention, in a field effect type device having a thin film-like active layer, there is provided a thin film-like semiconductor device comprising a top side gate electrode on the active layer and a bottom side gate electrode connected to a static potential, the bottom side gate electrode being provided between the active layer and a substrate.
According to another aspect of the invention, in a field effect type device having a thin film-like active layer, there is provided a thin film-like semiconductor device comprising a top side gate electrode on the active layer and a bottom side gate electrode (rear electrode) electrically connected to only one of a source and a drain of the field effect type device, the bottom side gate electrode being provided between the active layer and a substrate.
According to still another aspect of the invention, there is provided a thin film-like semiconductor device comprising a bottom side gate electrode (rear electrode) on a substrate having an insulating surface, a semiconductor layer having N-type and P-type impurity regions for covering the bottom side gate electrode, and two gate electrodes provided on the semiconductor layer, one of the last-mentioned gate electrodes being located out of the bottom side gate electrode. A p-type transistor is provided on the insulating surface and comprises an active region and a gate electrode provided on the active region. An n-type transistor is provided on the insulating surface and comprises another active region and another gate electrode provided on the another active region. The active region of only one of the p-type transistor and the n-type transistor is provided on the rear electrode. The rear electrode is kept at a potential of the source of the only one of the p-type transistor and the n-type transistor.
Preferably, the gate electrode of P-channel type transistor is located out of the bottom side gate electrode.
According to the invention, there is provided a method for producing a thin film-like semiconductor device, comprising the following steps: selectively forming a first semiconductor coating film, having a first conductive (conductivity) type, on a substrate having an insulating surface; forming a first insulating coating film on the first semiconductor coating film; forming a second semiconductor coating film for covering the first insulating coating film; forming a second insulating coating film on the second semiconductor coating film; forming at least two gate electrode portions on the second insulating coating film; dispersing impurities for the first conductive type into the second semiconductor coating film in a self-alignment manner relative to the gate electrode portions; and after the dispersing step, in a self-alignment manner relative to at least one of the gate electrode portions, dispersing impurities for a conductive (conductivity) type opposite the first conductive type in the second semiconductor coating film below which the first semiconductor coating film is not present.
According to the invention, there is provided a method for producing a thin film-like semiconductor device, comprising the following steps: forming, on a substrate having an insulating surface, a first conductive coating layer made of one selected from the group essentially consisting of semiconductor and metal; forming a first insulating coating film on the first conductive coating film; forming a first semiconductor coating film on the first insulating coating film; forming a second insulating coating film on the first semiconductor coating film; forming an etching mask material on the second insulating coating film; forming a hole in the etching mask material; forming a contact hole in the second insulating coating film in accordance with an isotropic etching process while using the etching mask material as a mask, that is, through an opening of the etching mask; forming a hole (an opening) in the first semiconductor coating film in accordance with an anisotropic etching process while using the etching mask material as a mask; and forming a hole (an opening) in the first insulating coating film in accordance with one of the isotropic etching process and the anisotropic etching process while using the etching mask material as a mask, thereby forming an electrode connected between the first conductive coating film and the first semiconductor film.